Commonly assigned U.S. Pat. No. 5,689,384 is incorporated for its showing of a timing based track following servo system for use with magnetic tape systems in which magnetic servo track patterns contain transitions recorded at more than one azimuthal orientation across the width of the servo track.
This invention relates to servo systems for positioning magnetic heads laterally with respect to longitudinal defined servo tracks or bands recorded on magnetic tape, and, more particularly, to recovery of lateral position once lateral position has been lost.
The reading and/or writing of data in tracks on magnetic recording tape requires precise positioning of a magnetic head. The magnetic head must be moved to, and maintained centered over, specific longitudinal data tracks, as the magnetic tape is moved longitudinally past the magnetic head. The magnetic head is translated between tracks in a lateral direction with respect to the longitudinal data tracks.
A servo system is employed to move the magnetic head to and position the magnetic head in the center of the desired data track or tracks, and to track follow the center of the desired data track or tracks. The data tracks are becoming increasingly smaller and closer together in order to increase the data track density and thereby increase data capacity of a given tape. Hence, it has become desirable to place the longitudinal defined servo tracks or bands at various locations across the full width of the tape, separated by groups of data tracks. This allows the servo tracks or bands to be close to the data tracks and limit offsets due to tape stretch, etc. This also allows a greater number of tracks to be employed due to the greater precision of the relationship between the servo tracks or bands and the data tracks.
An advantageous type of servo track is that of the incorporated ""384 patent, which is timing based. The magnetic servo track patterns contain transitions recorded at more than one azimuthal orientation across the width of the servo track, such that the timing of the servo position signal pulses derived from reading the servo pattern at any point on the pattern varies continuously as the head is moved laterally across the width of the servo track. The timing of pulses generated by the servo sensor is decoded to provide a speed invariant position signal used by the servo system to position the data heads over the desired data tracks.
Any one servo track may additionally have a plurality of indexed defined servo positions across the width of the servo track, called a servo band, thereby further increasing the number of data tracks that can be placed in the space between the servo bands. Herein, xe2x80x9cservo bandxe2x80x9d comprises a single position servo track or a servo track having a plurality of indexed servo positions.
So as to maximize the number of data tracks, the width of the servo bands is as narrow as practical, and the separation between the servo bands is as far apart as practical. An approach for maximizing the precision of the servo bands with respect to the data tracks is discussed in the above specification, and comprises employing a dual servo sensor for sensing the lateral position with respect to two of the defined servo bands, which are on opposite sides of the data tracks. The servo signals detected from the two defined servo bands may be averaged or separately detected for redundancy to better position the magnetic head at the data tracks. In one example, five defined servo bands may be placed across the width of the magnetic tape, and each servo band may have six index positions across the width of the defined servo band. Still further, the magnetic head may have a plurality of read/write transducers which are situated between the servo sensor transducers, so that a large number of data tracks may be provided for each index position.
Such servo systems typically employ compound actuators to provide both a large working dynamic band and high bandwidth. A typical compound actuator comprises a coarse actuator portion, such as a stepper motor, and a fine actuator portion, such as a voice coil motor, mounted on the coarse actuator portion. Thus, a data head can be translated between index positions and between servo bands over a full width of the magnetic tape using the coarse actuator portion, and can track follow the lateral movement of the track, using the fine actuator portion of the compound actuator. The fine actuator portion of the compound actuator typically follows the track guiding disturbances to position the magnetic head at the center of the desired data tracks. However, it has a very limited range of travel. The coarse actuator portion carries the fine actuator portion from index position to index position and between servo bands.
As the result, as the compound actuator moves the magnetic head between groups of data tracks, the associated dual servo sensor moves across an extended transverse span of data tracks, and the servo system lacks any servo signals from the servo bands for feedback for precision control. The stepper motor of the coarse actuator portion is typically driven forward or backward by step drive pulses. Thus, the number of pulses are counted as the coarse actuator is moved in order to estimate when the coarse actuator has moved a certain distance such that one or more groups of the data tracks have been crossed and the desired set of two servo bands has been reached, with the desire to place the dual servo sensor within the target servo band. Then, the position error signal (PES) is employed to further move the coarse actuator to the desired index position within the servo band. The fine actuator may then lock to the desired index position of the band and follow the index position. If the coarse actuator can move to the new absolute lateral position within some tolerance enough to detect the PES, no problem occurs in the sequence to acquire the target index position.
However, the coarse actuator is driven by the stepper motor by means of a lead-screw, and typically has no absolute position sensor to provide coarse actuator position feedback. The stepper motor is driven essentially in an open loop manner, by supplying step signals, and actual motor response to the step signals is not monitored. The coarse actuator may stick at some position on the way to the new absolute reference position due to mechanical reasons, such as gear friction or particle effects, or the coarse actuator may jump over the new absolute reference position due to non-linearity factors on the coarse actuator, such as spring force effects, distortion of the gear, and gear and lead-screw backlash. When these errors are large enough to move the servo read element out of the servo bands, the lateral position is completely lost to the track following servo system and the acquire fails.
A sensing system may be provided to monitor the absolute position of the magnetic head relative to the magnetic tape in real time, such as an optical sensor and related electronics, but requires mounting space and is costly. It may be possible to move the tape and the coarse actuator to xe2x80x9chome positionsxe2x80x9d and to then derive the home position locations, and then attempt to recover by moving the tape and the coarse actuator to the vicinity of the tape head when the lateral position was lost. However, this is time consuming and highly disruptive, and may be ineffective
An object of the present invention is to provide recovery of lateral position once lateral position has been lost, without the addition of an absolute position monitor.
Disclosed are a servo system and method for positioning a magnetic head laterally with respect to a plurality of parallel, longitudinal defined servo bands of a magnetic tape, the defined servo bands each separated by a plurality of data tracks parallel to the defined servo bands, the defined servo bands arranged in a longitudinally shifted pattern for identification, the magnetic tape drive having a drive system for moving the magnetic tape in the longitudinal direction at a nominal speed for reading and/or writing data on the magnetic tape.
The servo system for recovering lateral position comprises a dual channel servo sensor, such as a dual channel read head and detector, for sensing lateral position of the head with respect to two of the defined servo bands; a compound actuator having a fine actuator portion for translating the head laterally with respect to the defined servo bands, and a coarse actuator portion for translating the fine actuator portion laterally with respect to the defined servo bands; a home lateral position sensor for sensing whether the compound actuator is within a predetermined distance of an edge of the tape; a longitudinal position estimator for estimating the longitudinal position of the tape; and a servo controller.
The servo controller is coupled to the dual sensor, the compound actuator, the home lateral position sensor, the longitudinal position estimator, and the drive system, for operating the compound actuator to laterally position the dual servo sensor at two of the defined servo bands, and to track follow the same index position at each of the two defined servo bands, thereby positioning the head.
The servo controller responds to a loss of the lateral position, sensing the longitudinal position estimator estimated longitudinal position of the magnetic tape. If the sensed estimated longitudinal position of the magnetic tape is within a predetermined distance of an end of the tape, the servo controller operates the magnetic tape drive to continuously move the magnetic tape in a longitudinal direction away from the sensed end of the magnetic tape; else, determines the direction the drive was moving the magnetic tape and operates the drive to continuously move the tape in the opposite direction. The servo controller senses the home lateral position sensor to determine whether the compound actuator is laterally within a predetermined distance of the edge of the magnetic tape. If the compound actuator is within the predetermined distance of the edge of the magnetic tape, the servo controller operates the coarse actuator portion in a lateral direction to move away from the edge of the magnetic tape; else, operates the coarse actuator portion to move in a lateral direction toward the edge. The servo controller senses the dual servo sensor to detect any two of the defined servo bands; and, upon sensing any two of the defined servo bands, detects the longitudinal shift between two servo bands to identify which ones of the two defined servo bands have been detected.